Here are some thoughts...

I'm not sure a relay dedicated to photography would help much. Relay selection is not flexible enough in most clients. It might make sense to have a dedicated relay in combination with an app (or it could use a new KIND for photos are it's own thing on existing relays).

A marketplace for photographers could go into two directions: 
- A public stock photography site that sells images (see istockphoto.com)
- A private client gallery where photographers sell photos from photoshoots/weddings, etc. (e.g. https://pixieset.com/ and hundreds more).
For both usecases there are already a plethora of options.

There are a lot of photography communities already - it would be very hard to build a community on a new platform. Ideally we would have a generic nostr based forum software that could be customized to the usecase (e.g. like www.discourse.org, phpBB, ...). Someone should built that first :)

The Instagram type app is something that comes up every so often. The need for a photo focus app let me to build slidestr.net. The thing is, I'm not sure a generic social photo app will gain much traction today.

It worked in the past with Google+ (a lot of photographers joined there), Instagram (fixed square format + filters), 500px / flickr, ... but it needs some clear advantage 
over the existing solutions for people to switch.

Today most photography is done on mobile phones, i.e. I think people expect to use a (native) mobile app. All the PWA / web app stuff is to cumbersome for people.

A combination of the community and the app idea, could be a place for photography challenges (e.g. like https://52frames.com/albums/2024/week-42-on-the-ground/challenge ) where people post their work for daily/weekly/monthly challenges or projects regarding a specific topic. This would be a kind of learning community with image galleries.